1. Technical Field
The present disclosure relates to a technique of evaluating whether a speech sound has been heard in comfort. More specifically, the present disclosure relates to a technique of estimating an uncomfortable sound pressure with respect to a pure tone, in the context of “fitting” of a hearing aid or the like, i.e., adjusting an amount of amplification for each frequency of an external sound to arrive at a sound of a loudness which is appropriate to each individual user.
2. Description of the Related Art
In recent years, people who are suffering from hypacusia because of old age that need hearing aids are increasing in number. Their number is said to be about 20 million domestically, and about 500 million globally (as investigated by the Japan Hearing Instruments Manufacturers Association). Before beginning use of a hearing aid, “fitting” is required for adjusting the amount of sound amplification for each frequency in accordance with the auditory characteristics of the user.
For the sake of fitting, the user needs to visit a hearing aid shop several times to make readjustments because it is generally difficult to complete fitting in one time. One reason thereof is that an uncomfortable sound pressure (or uncomfortable loudness level: hereinafter referred to as “UCL”) cannot be correctly measured. In a test based on subjective reporting where the subject himself or herself must hear an actually-generated sound to tell whether it presents an uncomfortable sound pressure or not, the need for such loud sounds impose psychological stress and fatigue on the subject; therefore, a UCL is often determined through calculation from a hearing threshold level. However, a problem of this is that a UCL which is universally calculated from a hearing threshold level will not reflect individual differences.
Accordingly, techniques for estimating UCL by using an electroencephalogram, which reflects electrical activities of the brain, are being developed in the recent years. “Estimating uncomfortable loudness levels based on event-related potentials to triplets of auditory stimuli”, Shinobu ADACHI et al., Proceedings of the 51th Japanese Society for Medical and Biological Engineering, O1-10-1, 2012 (Hereinafter, Non-Patent Document 1) discloses a technique of estimating a UCL of a person with normal hearing with respect to each frequency, this technique using an electroencephalogram (auditory evoked potential) which is induced by triplet sounds of 80 dBHL or less, which is not loud. Since this technique utilizes an electroencephalogram of the subject when he or she hears a sound stimulation of a non-loud sound pressure for a short time, it is supposed to enable an individual UCL estimation.